The present invention relates to a frequency converter used in radio signal receiving apparatus, particularly to, a frequency converter capable of reducing noise components included in local oscillation signals.
A frequency converter shown in FIG. 1 has been well known as the conventional converter, which roughly comprises a high-frequency signal input circuit 10 for outputting a bias voltage and an inputted high-frequency signal such as a radio frequency -RF- which has undergone impedance and the like, a local oscillation signal input circuit 2 including bias adding means 28 or outputting a bias voltage and an inputted local oscillation signal supplied from local oscillator (not shown), and a multiplication circuit 30 for generating an output signal as an intermediate frequency -IF- signal by a multiplication of the high-frequency signal and the local oscillation signal supplied after each of bias voltage is respectively determined by the input circuits 10 and 2.
The local oscillation signal input circuit 2 inputs the local oscillation signal, adds a bias potential or current to the inputted local oscillation signal, and outputs a bias-added local oscillation signal. The input circuit 2 outputs not only supplied noises as they are, which are superposed on the local oscillation signal, but also original noises such as thermal noises occurring in the local oscillation input circuit 2. Accordingly, various noise components are included in the local oscillation signal.
There is described operation of the conventional frequency converter. A signal supplied to a terminal RF is inputted into a base terminal of a transistor Tr1 of the multiplication circuit 30 through the high-frequency input circuit 10. On the other hand, the local oscillation signal supplied to a terminal Lo is inputted into respective base terminals of transistors Tr2 and Tr3 constituting a differential pair through the local oscillation signal input circuit 2. Operation of the differential pair has been described in "Analysis and Design of Analog integrated Circuits" written by P.R.Gray and R.G.Meyer, in which a current inputted to common emitter terminal thereof is distributed to both of the transistors Tr2 and Tr3 on the basis of a potential difference between respective bases thereof, and collector currents of the transistors are thus outputted. The collector currents are converted into respective voltage outputs in a load circuit of the multiplication circuit 30.
The frequency converter using the transistor is differential pair causes the transistors to operate as a switch by a large amplitude of a voltage supplied to both base terminals thereof for purpose of a reduction of change of a conversion gain.
An output at this time can be expressed by an equation (i) as follows: EQU Vout(t)=K.times.F(t).times.{Irf(t)+Iee} (1),
where Irf denotes a high-frequency signal current outputted from a collector of the transistor Tr1, Iee denotes a bias current flowing to the collector of the transistor Tr1, F(t) denotes a function of which "1" and "-1" alternately appear at the same frequency as the frequency of the local oscillation signal, and K denotes a constant determined by the load circuit.
The function F(t) includes frequency components which are multiple of integer of the frequency of the local oscillation signal, and a rate of the components changes with a duty ratio.
As has been shown in the previous transcript C-82 of a lecture of a sprang conference in 1994 of an Institute of Electronics, Data and Communication Engineers of Japan, the current Irf is inputted into a frequency mixer after a bandpass filter (BPF) removes unnecessary signal components. Desired signals can be expressed by the term of "sin(2.pi.ft).times.Irf" in the equation (1), where the symbol f denotes a frequency of the local oscillation signal.
There has been described above the relationship between an output and input including a local oscillation signal and high-frequency signal. It is necessary to carefully design communication equipments especially receiver dealing with a fine radio signal for the purpose of avoiding a remarkable deterioration of a signal quality by noises generated in each of circuit elements. Even though filters prevent the high-frequency input signal in many receivers from a mixture of unnecessary signals and noises as described above, it has been considered that the local oscillation signal undergoes small influence noises because noise level is smaller than a level of the local oscillation signal. However, the noises in the local oscillation signal input circuit appear strong in the output so as to cause a signal quality to be deteriorated by the following reasons.
For the purpose of simplifying the description, noises will be described in this specification on the assumption that they are replaced by disturbance signals having a single frequency. Accordingly, the local oscillation signal and the disturbance signal are assumed to be inputted to the base input terminals of the transistors of the differential pair In the multiplication circuit. The differential pair can be regarded as a switch which changes over a current by positive and negative phases of the input signal. FIG. 2 is a characteristic diagram showing the case where the disturbance signal has a frequency twice as large as a frequency of the local oscillation signal. In the figure, a solid line shows a graph of the local oscillation signal, and dotted line and chain line show graphs of the product of the disturbance signal and minus one (-1), respectively. Accordingly, a zero-cross point of voltages between the bases of the differential pair is represented by a cross point of the solid and dotted lines (shown by symbol .circleincircle. in the figure), or by a cross point of the solid and chain lines (shown by symbol .largecircle. in the figure). As shown in the figure, when a phase of the disturbance signal changes, a time length in which an input of the differential pair is positive changes with the phase of the disturbance signal. When a frequency of the disturbance signal deviates from a value twice greater than the local oscillation signal, a phase relationship of both signals becomes to change little by little. Even when the amplitude of the disturbance signal changes, the zero-cross point moves. Accordingly, the frequency F(t) in the equation (1) is modulated by a pulse width modulation (PWM) by the amplitude and phase of the disturbance signal.
In many cases, the bias current Ice is set to be a value larger than the high-frequency signal current Irf. The product of F(t) and lee appears in an output Vout(t). In a homodyne receiver system in which output of the frequency converter is supplied to a low-pass filter, since the PWM signal is demodulated by the low-pass filter, these noises remarkably appear. Further, also in a heterodyne receiver system in which the output of the frequency converter is supplied to a bandpass filter, since the output (intermediate frequency) of the converter is set to a lower frequency of the order of one tenth (1/10) of the input high-frequency signal and the local oscillation signal in many cases, there is transmitted a noise component of a frequency oil-set by a value from twice greater than the frequency of the local oscillation signal to one twentieth (1/20) thereof to the output by the PWM modulation.
On the other hand, the disturbance signal having a frequency three times greater than that of the local oscillation signal, as shown In FIG. 3, is modulated by a pulse phase modulation and not by the PWM because the zero-cross point (shown by the symbol .largecircle. or .circleincircle. in the figure) moves in a substantially in parallel direction. Since the low-pass filter does not demodulate the pulse phase modulation signal, a noise component of a frequency three times greater than that of the local oscillation signal hardly has influence to the output.
This is not a phenomenon limited to a frequency twice or thrice greater than that of the local oscillation signal- A frequency which is multiple of even number including a multiple of zero of the local oscillation signal, appears in the output by the pulse width modulation. However, a frequency which is a multiple of odd number of the frequency of the local oscillation signal is not modulated because noise of multiple of the odd number is transmitted as the pulse phase modulated signal.
Here, the frequency which is multiple of zero of the frequency of the local oscillation signal is an alternative current waveform of a low frequency substantially close to a direct current, and is a frequency of the degree of a value less than one tenth (1/10) of the local oscillation frequency. In the case where the direct current as the disturbance signal is superposed on the local oscillation signal, a duty ratio of F(t) changes in correspondence with a very small amplitude near the direct current. For example, in a personal handy-phone (portable telephone) system --PHS--, since the receiver receives a high-frequency signal of 1.9 GHz, a local oscillation signal of 1.9 GHz is used in the homodyne receiver system. Even though a frequency which is multiple of zero greater than the local oscillation signal is substantially the direct current, a noise component of 50 kHz can be regarded as a disturbance signal having a frequency deviating by 50 kHz from that multiple of zero of the local oscillation signal. The noise component of multiple of zero appears in the output in a form of the PWM signal in the same manner as that in the case of a noise component having a frequency deviating by 50 kHz from a value two times greater than the local oscillation signal, namely, a noise component of 3.80005 GHz. Thus, such noise component is inputted to a channel selection filter. Since the channel selection filter has a low-pass characteristic of a cut-off frequency equal to or less than 150 kHz, the PWM signal is demodulated.
Since an electronic circuit necessarily has a parasitic capacitance, the higher the frequency becomes, the lower the noises are. Accordingly, noise having a frequency near the direct current give the largest influence on the output, and larger influence occurs in noises having a frequency near two times as large as the local oscillation frequency.